1. Field of the Invention
This invention relates to an electric motor having a permanent magnet in a rotor, such as Brushless DC motor, and more specifically, to an electric motor appropriate for the use as, for example, a driving source of a compressor of an air conditioner or the like, and capable of achieving a performance, size and production cost relative to its usage.
2. Description of the Related Art
In an electric motor such as Brushless DC motor, permanent magnets are embedded in a core of an inner rotor thereof, of which conventional examples are shown in FIG. 23 and FIG. 24. Incidentally, each drawing is a plane view of the inside of the electric motor shown from a plane perpendicular to the rotation axis thereof.
In the conventional example shown in FIG. 23, a rotor core 2 is disposed in a stator core 1 having, for example, 24 slots in which a field magnet rotates. The number of poles of the above electric motor is four, so that four permanent magnets 3 are arranged in the rotor core in accordance with the number of poles.
Each permanent magnet 3 is formed in a band plate shape of rectangular cross-section, and each pair of permanent magnets 3 as the south poles and the north poles is arranged across from each other along a direction perpendicular to a diameter line of the rotor core 2 on the outer circumference side of the rotor core 2. Each permanent magnet 3 is embedded in the rotor core 2 in a direction orthogonal to paper drawn with FIG. 23.
Between the two permanent magnets 3, a hole 4 as flux barrier is formed for avoiding short-circuiting and leaking the magnetic flux occurring between the adjacent permanent magnets. In this case, the hole 4 is represented as a triangle-shaped hole and located at each end of each permanent magnet 3. In the central portion of the rotor core 2, a center hole 5 is opened to pass a rotation shaft (not shown) therethrough.
In this point, when the magnetic flux distribution in a gap portion (between teeth of the stator core 1 and the permanent magnets 3) caused by each permanent magnet 3 is in a sine wave state, torque T of the electric motor is given as T=Pn{.PHI.a.multidot.Ia.multidot.cos .beta.-0.5 (Ld-Lq).multidot.Ia.sup.2 .multidot.sin 2.beta.}, where .PHI.a is an armature flux-linkage caused by the permanent magnet 3 on the d and q coordinate axes, Ld and Lq are the d-axis inductance and the q-axis inductance respectively, Ia is amplitude of an armature current on the d and q coordinate axes, .beta. is a lead angle of the armature current from the q axis on the d and q coordinate axes, and Pn is a pole-logarithm.
In the above expression, the first term expresses a magnet torque generated by the permanent magnets 3 and the second term expresses a reluctance torque generated by the difference between the d-axis inductance and the q-axis inductance. Refer to a treatise published in T. IEE Japan, vol. 117-D, No. 8. 1997 for further detail.
In the rotor core 2 shown in FIG. 24 as another conventional example, a permanent magnet 6 of arc-shaped cross-section is used, of which torque T is also given by the aforementioned expression.
Permanent magnets are embedded in number according to the number of poles in a rotor core. In most cases, the conventional art has used only a type of permanent magnet, for example, one of a ferrite magnet and a rare-earth magnet. Therefore, the flexibility in design is a low degree, and size and performance in torque, efficiency and so on are prone to be standardized; moreover production cost is directly determined by the permanent magnet used.
For example, where the permanent magnet 3 as a magnetic pole of the rotor core 2 shown in FIG. 23 is made of a rare-earth magnet, a smaller size and a higher performance can be achieved but the cost is high. On the other hand, where the permanent magnet 6 as a magnetic pole of the rotor core 2 shown in FIG. 24 is made of a ferrite magnet, the cost is lower but to acquire the equivalent performance to that of the rare-earth magnet, the diameter of the rotor core 2 needs to be increased.
The ferrite magnet is inexpensive and allows to form the permanent magnet in various configurations by reason of its ease of shaping, but the magnetic flux density is low, therefore hindering the reduction in size of the rotor core. On the other hand, although the rare-earth magnet has a high magnetic flux density and the reduction in size of the rotor core can be easy, the configuration of the permanent magnet is limited by the difficulties of shaping thereof. In addition, the rare-earth magnet has a higher cost than the ferrite magnet.
As described hereinbefore, in the conventional art, since a type of permanent magnet is used for magnetic poles in a rotor core, the range of choice in the performance, size and cost is limited, resulting in the difficulty in obtaining an electric motor appropriate for the required use.